use std::ffi::{OsStr, OsString};
use std::os::unix::process::CommandExt;
use std::process::Command;
use std::sync::OnceLock;

static APP_BACKEND: OnceLock<Option<OsString>> = OnceLock::new();

/// Called at process entry, before the toolkit or workers start threads.
pub(crate) fn configure_shell() {
    let backend = std::env::var_os("ICED_BACKEND");
    let use_shell_default = backend.is_none();
    APP_BACKEND.set(backend).expect("renderer configured once");

    if use_shell_default {
        // Small shell surfaces do not need a second GPU device and shader setup.
        // SAFETY: main calls this before starting any threads.
        unsafe { std::env::set_var("ICED_BACKEND", "tiny-skia,wgpu") };
    }
}

pub(crate) fn configure_app(command: &mut Command) {
    // Applications survive cleanup of a crashed desktop helper generation.
    command.process_group(0);
    if let Some(backend) = APP_BACKEND.get() {
        apply_app_backend(command, backend.as_deref());
    }
}

fn apply_app_backend(command: &mut Command, backend: Option<&OsStr>) {
    if let Some(backend) = backend {
        command.env("ICED_BACKEND", backend);
    } else {
        // Applications choose their own default rendering backend.
        command.env_remove("ICED_BACKEND");
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn settings_does_not_inherit_the_shells_automatic_software_renderer() {
        let mut command = Command::new("ferese-settings");
        command.env("ICED_BACKEND", "tiny-skia,wgpu");
        apply_app_backend(&mut command, None);
        assert_eq!(
            command.get_envs().find(|(key, _)| *key == "ICED_BACKEND").unwrap().1,
            None
        );
    }

    #[test]
    fn settings_preserves_explicit_renderer_preferences() {
        for backend in ["tiny-skia", "wgpu", "wgpu,tiny-skia"] {
            let mut command = Command::new("ferese-settings");
            apply_app_backend(&mut command, Some(OsStr::new(backend)));
            assert_eq!(
                command.get_envs().find(|(key, _)| *key == "ICED_BACKEND").unwrap().1,
                Some(OsStr::new(backend))
            );
        }
    }
}